Apparatus and methods for measuring thermally induced reticle distortion

ABSTRACT

An apparatus and method for measuring thermo-mechanically induced reticle distortion or other distortion in a lithography device enables detecting distortion at the nanometer level in situ. The techniques described use relatively simple optical detectors and data acquisition electronics that are capable of monitoring the distortion in real time, during operation of the lithography equipment. Time-varying anisotropic distortion of a reticle can be measured by directing slit patterns of light having different orientations to the reticle and detecting reflected, transmitted or diffracted light from the reticle. In one example, corresponding segments of successive time measurements of secondary light signals are compared as the reticle scans a substrate at a reticle stage speed of about 1 m/s to detect temporal offsets and other features that correspond to spatial distortion.

This application is a divisional of U.S. patent application Ser. No.13/277,085, filed Oct. 19, 2011, issued as U.S. Pat. No. 8,994,918,which claims the benefit of U.S. Provisional Patent Application No.61/405,592, filed Oct. 21, 2010, and U.S. Provisional Patent ApplicationNo. 61/443,630, filed Feb. 16, 2011, all of which are incorporatedherein by reference in their entireties.

FIELD

This disclosure pertains to, inter alia, reticles used for opticalpatterning of nanometer-sized features. More specifically, thedisclosure pertains to techniques for measuring dimensional changes anddistortion of a reticle primarily caused by reticle heating due toincident illumination.

BACKGROUND

Optical lithography may be used to pattern features on a substrate byilluminating a light-sensitive layer of the substrate at a wavelengthcomparable to a desired feature size. Such a technique is commonly usedin the semiconductor industry, for example, to pattern integratedcircuits on semiconductor wafers. State-of-the-art semiconductorlithography utilizes light of ultraviolet (UV) or deep ultraviolet (DUV)wavelengths, typically in the range of 150-500 nm, to pattern successivelayers of material, in order to form electronic structures havingfeature sizes in the submicron-to-nanometer range.

The process of transferring a pattern to a wafer substrate is analogousto a conventional photographic printing process. In the case of photoprinting, a light beam passes through a film “negative” and through oneor more enlarging lenses, to form an image on photosensitive paper,which is then chemically developed to produce a print of the image; inthe case of optical lithography, a light beam passes through a quartzphotomask, or “reticle” bearing a metallic circuit pattern, and throughone or more reducing lenses, to form an image in a photosensitivematerial on a target substrate. The substrate may then be etched orotherwise chemically processed to transfer the image to an underlyinglayer of material. Each flaw in a reticle thus has the potential toaffect mass production of a large number of electronic circuit chips.

Because the metal-patterned quartz reticle is exposed for long periodsof time to small-wavelength, high-energy light in close proximity to alight source, the reticle tends to absorb heat, causing thermalexpansion. If the thermal expansion occurs in a non-uniform manner,pattern dimensions on the reticle can become distorted. Non-uniformthermal expansion can occur, for example, if the density of the metalpattern is non-uniform across the reticle. Pattern density variationsmay be managed by establishing and enforcing circuit design rules duringproduction of the reticle. However, as integrated circuit feature sizesshrink, there is less tolerance for change in reticle dimensions. Inaddition, use of a double-patterning technique (in which the samematerial layer is patterned a second time with new features alignedin-between existing features) is becoming more widespread, which resultsin tighter manufacturing requirements for overlay budgets. Measurementand control of reticle distortion thus can provide a competitive edge byenabling pattern-overlay capabilities that have greater accuracy andprecision.

SUMMARY

The disclosure herein is directed to apparatus and methods for detectingand measuring thermally or mechanically induced reticle distortions insitu, thus enabling measurements of reticle distortion at the nanometerlevel. The techniques described use relatively simple optical detectorsand digitizing signal acquisition electronics that are capable ofmonitoring distortions in real time, while the lithography equipment isin operation. Both the magnitude and the direction of reticle distortioncan be measured at multiple locations on the reticle. Calibration of thedetector response can be done using existing stage control and metrologytechniques. Time-varying distortion of a reticle can be measured byreflecting or diffracting light focused to slit-like patterns ofdifferent orientations on the reticle as the reticle is scanned,detecting and recording the reflected or diffracted light, and comparinga segment of the recorded intensity with that measured earlier at thesame reticle stage position.

According to representative embodiments, one or more optical beams aredirected along a path incident to the surface of the reticle, which issecurely mounted to a moveable stage. A scan of the substrate can bemade by moving the reticle in a horizontal plane above the substrate.One disclosed embodiment allows for recording successive timemeasurements as the reticle scans the substrate. One or more detectorscollect secondary light signals that are scattered or reflected from achrome pattern on the reticle, or transmitted through the quartz reticleat locations where there is no chrome pattern. As the reticle scans,secondary intensity signals at the detectors vary with time, e.g., at afast reticle-stage speed of 1 m/s, the detector signal may change at thenanosecond (ns) level. Detected secondary signals, though they can bevery complex, are reproducible with stage position. Therefore,representative segments of the signals may be sampled and stored duringa scan, and corresponding segments from successive reticle scans may becompared to identify temporal offsets in the scan signals that indicatethermal or mechanical distortion. Other embodiments disclosed usevarious types of optical interferometry to detect reticle distortionbased on shifts in path length or phase of a diffracted signal.

In some examples, apparatus for in situ measurement of reticledistortion include a reticle scanning stage configured to retain areticle so that a pattern defined on the reticle is situated in areticle plane. At least one optical interrogation beam source isconfigured to provide an interrogation optical beam that is directed tothe reticle plane. A corresponding photodetector is situated withrespect to the reticle plane so as to receive at least a portion of theinterrogation optical beam from the reticle plane based on interactionof the interrogation optical beam and the reticle pattern, and produce acorresponding reticle characteristic electrical signal. A signalprocessing system is coupled to the at least one photodetector so as toreceive the reticle characteristic electrical signal, and based on theelectrical characteristic signal, provide an indication of a reticledeformation. In some examples, the signal processor is configured toreceive the reticle characteristic electrical signal as a function oftime within a scan of the reticle stage, and the indication of thereticle deformation is based on variation in the reticle characteristicelectrical signal as a function of time. Typically, the indication ofthe reticle deformation is based on variation in the reticlecharacteristic electrical signal as a function of time with respect to areference signal or a variation from a reticle characteristic electricalsignal obtained in a previous scan during exposure of the reticle. Inrepresentative examples, signal comparisons are based oncross-correlations. The reticle characteristic signal can be associatedwith direct detection, interferometric detection, or speckle detection.

Methods for detecting reticle distortion comprise directing at least oneoptical beam to be incident on the reticle as the reticle undergoes ascanning motion and detecting at least one segment of a secondary lightsignal associated with an interaction of the at least one optical beamand the reticle during at least a portion of a scan interval. Based onthe detected segment, an extent of a reticle deformation is determined.In some examples, the extent of reticle deformation is based on acomparison of the segment with a reference segment obtained from a priorscan of the reticle. Typically, an exposure beam is directed to thereticle during acquisition of the detected segment. In some examples,the at least one segment is detected interferometrically, and areference optical beam is provided. In other examples, the at least onesegment is detected based on detected speckle. In some embodiments, anextent of a reticle deformation is determined based on comparisons ofthe detected segments. In some examples, the comparisons are based oncorrelation of detected segments. In additional examples, at least onetemporal offset is determined based on the correlation of detectedsegments, and the determined extent of reticle deformation correspondsto a spatial displacement associated with the reticle.

These and other features and aspects of the disclosed technology are setforth below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a representative apparatus formonitoring reticle distortion.

FIGS. 2A-2B are plots of detected interrogation beam intensities asfunctions of time during segments of adjacent reticle scan intervals.

FIG. 3 is a block diagram of a representative method of measuringreticle distortion in situ based on detected portions of an opticalinterrogation beam acquired during reticle scanning.

FIG. 4 is a block diagram of a representative method of comparingdetected portions of an optical interrogation beam based oncorrelations.

FIG. 5 is a schematic diagram of a representative apparatus formonitoring reticle distortion based on diffraction of an interrogationoptical beam by a portion of a pattern defined on a reticle.

FIG. 6 is a plot of relative diffracted optical intensity as a functionof diffraction angle.

FIG. 7 is a schematic diagram of a representative apparatus formonitoring reticle distortion based on interferometric detection of aninteraction of an interrogation optical beam with a reticle.

FIG. 8 is a graph illustrating a representative interferometric signalobtained using an apparatus such as that of FIG. 7.

FIG. 9 is a schematic diagram of a representative apparatus formonitoring reticle distortion based on interferometric detection of aninteraction of an interrogation optical beam with a reticle in which areference beam is obtained by reflection of a portion of aninterrogation optical beam.

FIG. 10 is a schematic diagram of a speckle interferometry apparatus fordetecting reticle distortion using two laser beams that are coincidenton a reticle pattern.

FIGS. 11A-11C illustrate representative slit illumination patterns thatpermit determination of distortion direction and magnitude.

FIGS. 11D-11I illustrate representative diamond-shaped illuminationpatterns that permit determination of distortion direction andmagnitude.

FIG. 12 illustrates acquisition of an electrical signal based ondetection of optical interrogation signal.

FIG. 13 is a schematic diagram of an immersion microlithography system,which is a first example of a precision system including a stageassembly as described herein.

FIG. 14 is a schematic diagram of an extreme-UV microlithography system,which is a second example of a precision system including a stageassembly as described herein.

FIG. 15 is a process-flow diagram depicting exemplary steps associatedwith a process for fabricating semiconductor devices.

FIG. 16 is a process-flow diagram depicting exemplary steps associatedwith a processing a substrate (e.g., a wafer), as would be performed,for example, in the process shown in FIG. 15.

DETAILED DESCRIPTION

As used in this application and in the claims, the singular forms “a,”“an,” and “the” include the plural forms unless the context clearlydictates otherwise. Additionally, the term “includes” means “comprises.”Further, the term “coupled” does not exclude the presence ofintermediate elements between the coupled items.

The systems, apparatus, and methods described herein should not beconstrued as limiting in any way. Instead, the present disclosure isdirected toward all novel and non-obvious features and aspects of thevarious disclosed embodiments, alone and in various combinations andsub-combinations with one another. The disclosed systems, methods, andapparatus are not limited to any specific aspect or feature orcombinations thereof, nor do the disclosed systems, methods, andapparatus require that any one or more specific advantages be present orproblems be solved. Any theories of operation are to facilitateexplanation, but the disclosed systems, methods, and apparatus are notlimited to such theories of operation.

Although the operations of some of the disclosed methods are describedin a particular, sequential order for convenient presentation, it shouldbe understood that this manner of description encompasses rearrangement,unless a particular ordering is required by specific language set forthbelow. For example, operations described sequentially may in some casesbe rearranged or performed concurrently. Moreover, for the sake ofsimplicity, the attached figures may not show the various ways in whichthe disclosed systems, methods, and apparatus can be used in conjunctionwith other systems, methods, and apparatus. Additionally, thedescription sometimes uses terms like “produce” and “provide” todescribe the disclosed methods. These terms are high-level abstractionsof the actual operations that are performed. The actual operations thatcorrespond to these terms will vary depending on the particularimplementation and are readily discernible by one of ordinary skill inthe art.

In some examples, optical radiation is directed to and from a reticle orother surface. As used herein, optical radiation refers toelectromagnetic radiation in a wavelength range of between about 5 nmand 2000 nm. In some examples, a reticle is irradiated with opticalradiation in a first wavelength range in order to produce lithographicpatterning, and a second wavelength range for interrogation of reticledeformation. For convenience, optical fluxes or beams intended forlithographic patterning are referred to herein as exposure fluxes orexposure beams. Optical fluxes or beams intended for reticle assessmentare referred to as interrogation fluxes or interrogation beams. Eitherof such fluxes can be a narrow band or wide band flux, and polarized,partially polarized, and unpolarized fluxes can be used. In someexamples, the fluxes are spatially and/or temporally coherent, and areprovided by one or more lasers.

In some disclosed examples, reticle deformation such as that produced byheating associated with irradiation by a patterned beam can be assessed.As used herein, distortion refers to any reticle pattern shape or sizechange, or any shape, size, or spacing change of pattern elements on areticle. Such distortion can be uniform or non-uniform, and thedistortion can be isotropic or anisotropic. Typically, pattern exposuresare made by moving a reticle with respect to a surface to be exposed tothe reticle patterns. Such motion can be continuous, stepped, or acombination thereof. In some examples, a reticle is interrogated with aninterrogation beam during scanning or with the reticle at rest.

FIG. 1 illustrates a representative reticle distortion measurementsystem 100 situated for in situ reticle distortion measurements. Asshown in FIG. 1, a patterned reticle 102 is secured to a reticle stage104 that is configured to be stepped, scanned or otherwise moved. In theexample of FIG. 1, the reticle 102 can be scanned substantially in ahorizontal plane in a y-direction based on xyz-coordinate axes 108. Anx-axis extends out of the plane of FIG. 1, but is not shown. Typically,a chrome pattern 110 or other pattern on the reticle 102 is irradiatedby an exposure source 112 in a wavelength range suitable for patterntransfer, and projection optics 114 form an image of the exposed portionof the reticle 102 on a sensitized substrate such as a photoresistcoated wafer 105 that is situated on a wafer stage 107. For convenientillustration, other components associated with pattern-transfer from thereticle 102 to a wafer are either omitted or are shown schematically inFIG. 1. Additionally, the reticle 102 is shown as a reflective element.In other configurations the reticle is transmissive, with light from theexposure source 112 passing through the reticle to the projection optics114.

An interrogation optical beam 111 from an interrogation beam source 118is directed along an interrogation beam axis 116 to the reticle 102, anddetectors 120, 122, 124 are situated along respective detection axes128, 130, 132 so as to capture portions of the interrogation beam 111that are reflected, refracted, diffracted, transmitted or otherwisedirected as “secondary” optical signals, to a respective detector. Suchsecondary signals are also referred to as reticle modulatedinterrogation beams. A plurality of detectors is shown in FIG. 1, butone or more detectors can be used, and detector position can be selectedas convenient, and may be dependent on the reticle 102. Theinterrogation beam 111 can be a coherent or incoherent beam, buttypically coherent optical radiation is convenient. The interrogationbeam source 118 can include a laser, lamp, light emitting diode or otheroptical radiation source. For example, a laser source that produces alaser beam in a visible wavelength range (typically between about 400 nmand 700 nm) can be used, or portions of the optical radiation used inpattern transfer and provided by the exposure source 112 can be used. Insome examples, reticle distortions can be associated with diffractiveeffects, so that longer wavelengths with greater diffraction angles canprovide superior results. The locations and orientations of theinterrogation beam axis 116 and detector axes 128, 130, 132 are providedas examples only, and other arrangements can be used as may beconvenient. The interrogation beam 111 generally propagates in a fixeddirection to a reticle surface and the reticle 102 is scanned throughthe interrogation beam 111.

The detectors 118, 120, 122 can be provided with respective collectionlenses 129, 131, 133 to, for example, increase collection efficiency ofradiation from the interrogation beam. The detectors 120, 122 aresituated to receive diffracted, scattered, or specularly reflectedportions of the interrogation beam 111 and the detector 124 is situatedto receive portions of the interrogation beam 111 that are scattered,diffracted, or otherwise transmitted by the reticle 102. In FIG. 1, thedetector 120 is provided with an actuator 121 configured to displace thedetector 120 along the axis 128 in order to compensate z-directeddisplacements of the reticle 102 that can be made to increase anapparent depth of field (DOF) in imaging the reticle 102 onto thesubstrate 105. Other detectors can be similarly provided with actuators.In the example of FIG. 1, three detectors are provided, but one, two,three or more detectors can be used as preferred. The collection lenses129, 131, 133 are shown as single element lenses, but in other examples,collection optics can be provided that have multi-element lenses,mirrors, or other reflective or refractive optical elements orcombinations thereof, and optical filters or polarizers can be used toattenuate unwanted optical radiation that can reach the detectors. Forexample, filters can be provided to block or attenuate any scattered orother portions of the exposure beam that might otherwise reach one ormore of the detectors 128, 130, 132.

The detectors 128, 130, 132 are coupled to a data acquisition unit 134that generally includes optical detection electronics such asamplifiers, electrical filters, analog-to-digital convertors and othercomponents configured to process detector output signals and producecorresponding electrical signals that can be stored in a memory asrecorded signals. The data acquisition unit 134 can be configured toinclude a computer readable medium such as memory for storage ofrecorded signals or for communication of the recorded signals to acontroller 136 for analysis and processing. The controller 136 can beprovided as a dedicated control computer, a desktop computer, a laptopcomputer, or other portable or non-portable processing system. Thecontroller need not be co-located with the optical system and exposureand interrogation sources, but can be coupled so as to communicate via alocal or wide area network using, for example, an ethernet or othernetwork protocol.

The controller 136 is also coupled so as to control reticle andsubstrate positioning such as scanning or stepping of the reticle 102and/or the substrate, typically via corresponding stages such as thereticle stage 104. The controller 136 can be provided as a computer orother processor that includes computer executable instructions forreticle and substrate positioning, scanning, data acquisition, andprocessing as well as communicating reticle assessments to a user. Thecontroller 136 typically includes storage devices or storage media suchas, for example, hard disks, RAM, ROM, or other computer memory that isconfigured to store data such as recorded signals andcomputer-executable instructions for operation and control.

The controller 136 is configured to store recorded detector signalsassociated with the detectors 128, 130, 132 with reference to reticlepositions. Typically, the recorded detector signals are referenced toreticle scan times so that recorded signals are available as function oftime during reticle scanning. Reticle scanning is generally repetitive,i.e., the same portions of the reticle 102 are exposed in sequentialscans, and accordingly, recorded detector signals are available forsequential scans. Scan-to-scan variations in recorded detector signals(or variations from initial measurement prior to any scanning) can beused to detect or assess reticle changes during scanning, such asreticle distortions due to thermal effects produced by irradiation withthe exposure beam. If successive scans are entirely repetitive,electrical signals from successive scans will be compared at the sameelapsed time from scan initiation. However small changes to the scanningprofile may occur, so that the reticle may be at slightly differentlocations at the same elapsed time for successive scans. The controller136 can be used to correct the elapsed times so that the reticle isinterrogated at the same reticle position.

In the example of FIG. 1, the detectors 128, 130, 132 are configured toproduce electrical signals based on received optical intensities fromthe reticle 102 associated with the interrogation beam 111. In otherexamples discussed below, the detectors are configured to produceelectrical signals based on optical interference such as dual beaminterferometry or speckle. In some cases, each or selected ones ofmultiple detectors can be configured to provide one or more electricalsignals based on direct detection, speckle, or beam interference.

FIGS. 2A-2B show example detector signals I_(n), I_(n+1) recorded duringidentical segments of consecutive scans. Times t_(n) and t_(n+1) are thetimes during respective scans at which recording of the segmentsstarted, and are generally selected so that detector signals areassociated with exactly the same regions of the reticle for the twoscans, barring reticle distortion. As shown in FIGS. 2A-2B magnitudes ofdetector signal intensities associated with a selected detector areplotted as a function of elapsed time from respective scan initiationtimes t_(n), t_(n+1), respectively, for an n^(th) and an n+1^(st) scan,as intensity curves 202, 204. In this example, a recorded signal isobtained for each of these scans individually, but a continuouslyrecorded signal can be divided into portions (i.e., signal segments)that can be associated with particular scans and elapsed time withinsuch scans.

In the absence of any deformation in the reticle or other variation inscans, detector signals such as those graphed in FIGS. 2A-2B should besubstantially the same. However, as shown in FIGS. 2A-2B, the curves202, 204 are horizontally offset by a temporal offset time Δt whichindicates reticle changes have occurred between the corresponding scans.Because thermal effects produced by exposure beam heating can berelatively small (even if larger than desirable for high resolutionlithography), curves such as the curves 202, 204 tend to be similar inappearance. However, a processing system such as the controller 136 canbe configured to process two or more recorded signals and, based ondifferences in corresponding signals, provide an indication of reticlechanges.

For recorded signals that exhibit a temporal offset, an estimate of acorresponding reticle pattern feature shift in a scan direction can beobtained, based on a reticle scan speed V and the temporal offset Δt asa product VAt. For example, if V=1 m/s, a temporal offset of 1 nscorresponds to, and indicates, a reticle displacement of 1 nm. Atemporal offset of 1 ns is directly resolvable if detector intensitiesare recorded at sampling rates of 1 Gsample/sec, but acquisition atlower sampling rates can be used and temporal offsets determined byinterpolation or other processes. However, temporal offset determinationcan be limited by both the stage speed V and the spatial resolution δxof collection lenses/detector combinations. For example, a secondarybeam such as a diffracted or reflected portion of the interrogationradiation originating at a reticle is received at a point detector orslit detector with slit orientation normal to a reticle stage traveldirection for a duration of approximately δx/V. If the point spreadfunction at a detector is about 1 μm and the reticle stage speed isabout 1 m/s, the secondary beam can persist on the detector forapproximately 1 μs. Increasing the detector slit size worsens the timeresolution so that small area detectors can be preferred, especially fortemporal resolutions approaching 1 ns. In addition, when evaluatingdistortion based on an interrogation beam, reticle-scanning systemirregularities can appear as reticle distortions and mechanically stablescanning is preferred so that the reticle stage location may berepeatably determined. However, repetitive errors (i.e., repeatableerrors) can be identified as such, and discarded in assessing exposureinduced reticle distortions.

Determination of reticle distortion based on representative recordeddetector signal segments I_(n) and I_(n+1) is illustrated in FIG. 3.Reticle irradiation with both pattern transfer and interrogation beamsis initiated at 300, and interrogation signal segments I_(n) and I_(n+1)are recorded at 301 for an n^(th) and an n+1^(th) scan. Typically, suchsignal segments are based on portions of the scan duration. At 302, thesignal segments are compared. If changes are detected at 303, distortionis noted as detected at 307. If distortion is determined to exceed alimit such as a warning limit at 308, a warning is provided, patterntransfer is halted, or system operating parameters such as, e.g., scan,reticle height, wafer height, or focus are adjusted at 310. Otherwisesignal segments for a different value of n are obtained at 312 and thedistortion evaluation is repeated. Signal segment comparison can bebased on signal segment amplitudes, phases, temporal offsets, shapes, orother signal characteristics.

The types of signal differences in signal segments can depend on theshapes, sizes, orientations, and densities of pattern features on thereticle, detector position(s), the types of optical signals received bydetectors, and types of reticle deformation. For example, if a reticleexperiences a thermal expansion or contraction, the relative position ofpattern features can change from scan to scan. Such relative positionchanges can appear as temporal offsets for detector signal segmentsassociated with different scans such as shown in FIGS. 2A-2B. Suchdetector signal segments can exhibit other differences associated withreticle deformations. For example, detector signal segments can vary asa function of time corresponding to differing reticle pattern featurespacings as a function of exposure. However, the disclosed methods arenot limited to any particular type or reticle change or deformation.

Temporal offsets or other changes in signal segments can be convenientlyassessed by processing signal segments to obtain cross-correlations. Tothis end, a general purpose controller (such as the controller 136 ofFIG. 1) can be configured as a signal processor or a separate, dedicatedsignal processor can be used. For example, a processor such as a generalpurpose or dedicated processor can be provided with computer-executableinstructions for such cross-correlations. In some examples,computer-executable instructions are provided for obtaining Fouriertransforms of recorded signals so that cross-correlations of first andsecond signal segments can be conveniently determined based on productsof Fourier transforms associated with the signal segments. Thecross-correlation can then be found based on an inverse Fouriertransform of the product. Such an approach can provide more efficientcomputation. In a typical method as shown in FIG. 4, recordedinterrogation signals such as I_(n) and I_(n+1) are obtained at 402, anda cross-correlation is calculated at 404. At 406 the cross-correlationis evaluated, and at 408 possible distortion (or a lack of distortion)is reported. In some examples, the cross-correlation provides anestimate of a magnitude and direction of a pattern displacement.Although in this example sequential interrogation signals for an n^(th)and an (n+1)^(th) scan are used, non-sequential scans can be used, andat least one signal can be associated with a reticle that is notreceiving an exposure beam, or that is otherwise thought to be free ofthermal or other distortions.

Generally scan-to-scan variations in detected portions of theinterrogation beam associated with distortion are small. In digitallyrecording such signals, it can be preferable to record a differencebetween a detected signal and a reference signal. The reference signalcan be recorded and then provided to a digital to analog converter(DAC). Referring to FIG. 12, an electrical signal associated with aportion of an interrogation beam detected at a photodetector is providedto a differential amplifier 1206 from, for example, a photodetectoramplifier 1202. The differential amplifier 1206 as illustrated is basedon an operational amplifier 1207, but other amplifiers could be used. Adigital reference signal source 1204 is coupled to a DAC 1208 whoseanalog output is coupled to the differential amplifier 1206. Thedifferential amplifier 1206 produces a difference signal based on adifference between the DAC output and the analog interrogation beamsignal. The difference signal is digitized by an Analog-to-Digitalconverter (ADC) 1210 and stored in a memory 1212. As shown in FIG. 12,the ADC 1210 includes interleaved ADCs 1210A, 1210B that can provideincreased temporal resolution based on phase difference in acquisitionsignals from a clock source 1210C, but a single ADC can be used. Asnoted above, recording optical signals are referenced to scan times, butscan time references are not shown in FIG. 12 for clarity.

Diffractive Interrogation Signals

In some examples, reticle pattern features can produce detectablediffracted optical beams in response to an input interrogation beam.Magnitudes of scattered or reflected portions of the interrogation beamare generally based on pattern density within an irradiated region ofthe reticle. Small, repetitive chrome patterns that have a substantiallyconstant pattern density may fail to produce noticeable changes indiffracted light intensity as scanned through an interrogation beam, andprovide limited contrast which may make feature identification anddistortion detection difficult. However, in scanning reticle areas inwhich pattern density in the irradiated reticle region changessubstantially, (e.g., when a feature having a large chrome surface area,such as an electrical contact pad, is exposed), changes in diffractionpatterns and diffracted beam intensities associated with reticledeformation can be more readily detected. For applications in whichdiffracted interrogation beams are to be detected, optical detectors canbe situated away from directions associated with reflection ortransmission of an interrogation beam. In general, diffraction intensityremains substantially constant with scanning of a diffractiongrating-like pattern feature through the interrogation beam, sostage-motion effects do not interfere with obtaining diffractionmeasurements in this manner. Diffracted light scattering intensity froma given reticle pattern generally depends with the wavelength of theinterrogation beam, and diffraction angles are proportional towavelength so that longer wavelengths can be preferred.

Diffraction from a one-dimensional grating is described by the gratingequation:mλ/p=n _(d) sin θ_(m) −n _(i) sin θ_(i),wherein m is a diffraction order m=0, ±1, ±2, . . . , λ is wavelength, pis a grating pitch (i.e., spacing of features), θ_(i) is an angle ofincidence, θ_(m) is a diffraction angle for an m^(th) diffraction order,nd is an index of refraction of the diffraction medium, and n_(i) is anindex of refraction of the medium of incidence. Because either theincident flux, or the diffracted flux, or both may pass through thereticle, both n_(d) and n_(i) may be greater than 1.0. If the spacingbetween grating slits p is less than λ/(max(n_(d), n_(i))+n_(i) sinθ_(i)), the only substantially diffracted order is m=0, so n_(d) sinθ_(m)=n_(i) sin θ_(i), and little or no diffractive modulation of theintensity signal occurs. However, when conditions permit non-zerodiffraction orders, a diffraction pattern is created, a portion of whichcan be intercepted by a detector, typically placed so as not to receivespecular reflections or direct transmission.

FIG. 5 illustrates a portion of an apparatus configured to detect orassess reticle deformations by detecting diffracted portions of aninterrogation beam. As shown in FIG. 5, a reticle 502 is situated onstage 504 that is coupled to a controller 506 that provides suitableelectrical signals for scanning of the stage 504. An interrogationsource 508 directs an interrogation beam 510 to the reticle 502 and inparticular to a portion 512 of a pattern defined on a surface of thereticle 502. As shown in FIG. 5, the pattern 512 is on a back surface514 of the reticle 502 but can be on a front surface 516 instead. Theinterrogation beam 510 is refracted at the front surface 516 and thendiffracted at least in part into a particular diffraction order thatpropagates along an axis 517 to a detector 522. An angle of incidenceθ_(i) and an angle of diffraction θ_(d) satisfy the diffractioncondition set forth above. Axes 526, 528 are associated with reflectionat the front surface 516 at an angle θ_(r) and the back surface 514 ofthe reticle 502. The detector 522 can be situated so as to be displacedfrom the axes 526, 528 so as to avoid detecting reflected portions ofthe interrogation beam, one or more additional detectors can be providedfor detecting reflected portions, or the detector 522 can be situated soas to receive both reflected and diffracted portions. Various collectionand beam forming optics such as lenses, mirrors, and diffractive opticalelements can be used, and filters such as holographic or dielectricfilters, polarizers, or absorptive filters can be used to attenuateundesired radiation from reaching a detector. Although the reticle 502is shown with a pattern on a back surface, a reticle can include a frontsurface pattern that is directly exposed to an interrogation beam,without transmission by a reticle substrate.

Diffraction of an interrogation beam by a periodic pattern similar to adiffraction grating on a reticle can be described based on an electricfield strength U(x₀, y₀) in a far field approximation as:

${{U\left( {x_{0},y_{0}} \right)} = {a\;\sin\;{c\left( \frac{y_{c}a}{\lambda\; z_{0}} \right)}\frac{\sin\left( \frac{N\;\pi\; y_{0}d}{\lambda\; z_{0}} \right)}{\sin\left( \frac{\pi\; y_{0}d}{\lambda\; z_{0}} \right)}{\exp\left( {{- {\mathbb{i}}}\;\frac{2\pi\; y_{0}{C(t)}}{\lambda\; z_{0}}} \right)}}},$wherein d is a grating slit spacing (d=1/p), a is slit width, λ is anirradiation wavelength, N is a number of irradiated slits, x₀, y₀, andz₀ are spatial coordinates, C(t) is phase factor, and constant factorsare omitted. Scanning of a periodic pattern that produces such a fieldstrength does not necessarily produce substantial variations in U(x₀,y₀), although a number of slits N irradiated can vary by one duringscanning. A simplified diffraction pattern 602 is illustrated in FIG. 6,wherein λ=500 nm, slit width a=1 μm and slit spacing d=1.752 μm and anumber of slits N=10. A detector can be situated so as to detect anyportion of this diffracted intensity to permit reticle distortiondetection.Phase-Sensitive Interrogation Signals

Diffraction based interrogation as described above is based on detectedintensities, i.e. |U(x₀, y₀)|², and is not dependent on the phase termC(t). As the reticle is scanned, the number of slits irradiated canchange, but the resulting intensity modulation of the diffracted opticalsignal is generally weak. In some cases, the phase factor C(t) can beused to detected reticle distortions. C(t) is based on a position of areticle pattern such as the pattern 512 with respect to the detector 522at time t. Phase information such as C(t) can be captured by interferingthe diffracted optical intensity with a second coherent beam as shown inFIG. 7.

With reference to FIG. 7, an interrogation beam source 708 directs aninterrogation flux to a beam splitter 709 that produces a firstinterrogation beam 714 and a second interrogation beam 716. The firstinterrogation beam 714 is directed to a reticle 702 on which a pattern712 is defined, and a diffracted, reflected, or other portion of thefirst interrogation beam is directed to a detector 722. The second beam716 is directed to the detector 722 so as to interfere with the portionof the first interrogation beam that reaches the detector 722. Theinterrogation beam source 708 provides an interrogation beam havingsufficient coherence so that interference is produced at the detector722.

The reticle 702 is secured to a moveable stage 704 that is scanned basedon a scanning signal provide by a controller 706. The controller 706 isalso coupled to the detector 722 so that optical interference patternscan be recorded as a function of time during reticle scanning forsubsequent processing. One or more collection optical elements (e.g.,lenses, mirrors, and the like) may be placed in front of the detector722 to, for example, increase radiation collection efficiency.Collection elements can be single element lenses, but in other examplescollection optics can be provided with multi-element lenses, mirrors, orother reflective or refractive optical elements or a combinationthereof, and optical filters or polarizers can be used to attenuateunwanted optical radiation that can reach the detector 722. The firstand second interrogation beams can be provided by dividing a single beaminto two beams, or separate sources which are coherently related can beused.

Total interrogation beam amplitude at the detector 722 at a height yoabove the reticle plane can be expressed as:

${{U_{h}\left( {x_{0},y_{0}} \right)} = {{a\;\sin\;{c\left( \frac{y_{0}a}{\lambda\; z_{0}} \right)}\frac{\sin\left( \frac{N\;\pi\; y_{0}d}{\lambda\; z_{0}} \right)}{\sin\left( \frac{\pi\; y_{0}d}{\lambda\; z_{0}} \right)}{\exp\left( {{- {\mathbb{i}}}\;\frac{2\pi\; y_{0}{C(t)}}{\lambda\; z_{0}}} \right)}} + {A\;{\exp\left( {{- {\mathbb{i}}}\;\frac{2\pi\; y_{c}\sin\;\phi}{\lambda}} \right)}}}},$wherein a, N, λ, d, and C(t) are defined above, A is a constant, and φis an angle between the first and second reference beams propagating tothe detector 722. The corresponding intensity at the detector 722 isI=|U(x₀, y₀)|², which is given by:

${{I_{h}(t)} = {D^{2} + A^{2} + {2{AD}\;{\cos\left( {\frac{2\pi\; y_{0}}{\lambda}\left( {\frac{C(t)}{z_{0}} - {\sin\;\phi}} \right)} \right)}}}},$wherein

${D = {a\;\sin\;{c\left( \frac{y_{0}a}{\lambda\; z_{0}} \right)}\frac{\sin\left( \frac{N\;\pi\; y_{0}d}{\lambda\; z_{0}} \right)}{\sin\left( \frac{\pi\; y_{0}d}{\lambda\; z_{0}} \right)}}},$and wherein the sinc function is defined as sinc(x)=sin(πx)/πx. Thus, asthe stage 704 moves and scans the reticle 702, C(t) steadily increases(or decreases) and the intensity changes cosinusoidally. The detector722 receives this optical intensity and produces a correspondingelectrical signal that can be recorded.

An example of a detected signal 800 captured at an interferometricdetector such as the detector 722 is shown in FIG. 8. In this example, adetector signal was estimated with φ=10°, y₀=0.1 m, and z₀=1.0 m. Underthese conditions, excellent contrast can be achieved. For a stage speedV=1.0 m/s, the signal changes at the sub-microsecond level, which issufficient to resolve sub-nm range distortion. Such a detected opticalsignal can be referred to as “hologram contrast” signal, because it isobtained with a layout similar to a layout for making holograms.

Under certain conditions, a reference interrogation beam can be providedas a reflected beam from a reticle surface. With reference to FIG. 9, aninterrogation optical beam 910 is directed along an irradiation axis 912to a reticle 902 that is secured to a moveable stage 904. Portions ofthe interrogation optical beam are scattered, diffracted, or reflectedfrom a periodic pattern 908 on the reticle 902, and are directed to anddetected by a detector 916. A portion of the interrogation optical beam910 is reflected at a reticle surface 911 and directed to the detector916 by a reflector 915. At the detector 916, the reflected interrogationbeam 918 interferes with the portion of interrogation beam received fromand modulated by the pattern 908. Additional optical elements can beprovided as discussed above, but are omitted from FIG. 9 for clarity.

Speckle Detection

Changes in speckle patterns from scan to scan can also be used to detectreticle deformations. Small variations in surface topography such assurface roughness can produce a high contrast speckle pattern whenexposed to a coherent light flux. The granularity of a speckle patternis typically comparable to the resolution of an imaging optical system.Thus, an imaging optical system having approximately micron resolutioncan produce high contrast speckle patterns with intensity variation on amicron scale. Therefore, the methods described above with respect toFIGS. 1-4 can be used with speckle-based signals to determinedistortions.

Pattern Features for Distortion Determination

While reticle distortion can be assessed based on pattern features to betransferred to a sensitized substrate in the lithographic process,specialized distortion detection features can also be provided for thispurpose. For example, for detection of reticle distortion in a scandirection (e.g., a y-direction) as shown in FIG. 11A, a slitillumination pattern 1100 extending along an x-axis can be used. It canalso be advantageous to measure distortion in a direction orthogonal tostage motion (i.e., the x-direction). If a reticle stage has even asmall motion, or a “short stroke” in the x-direction, the methodsdescribed above are applicable. However, in practice, a shortx-direction stroke may preclude reaching a sufficiently high stagevelocity (for example, 1 m/s) for obtaining sufficiently reliabledistortion measurements. A short x-direction stroke may also interferewith normal lithography operation. An alternative method of determiningboth x- and y-components of distortion while limiting stage motion to along-stroke y-direction can use successive measurements of thedistortion of test slit illumination patterns or similar illuminationpatterns that are oriented in different directions. Representativepatterns are illustrated in FIGS. 11A-11C which also depictdisplacements of various test slit patterns at different orientations,corresponding to a reticle pattern distortions in different directions.

Referring to FIG. 11A, a rectangular slit illumination pattern (referredto as “slit 1”) is elongated along the x-direction and has a long sidethat is perpendicular to a scan direction (y-direction). After thereticle experiences some distortion, the slit illumination pattern isshown as a +y-displaced slit pattern 1110, a +x-displaced slit pattern1112, diagonally displaced slit patterns 1114, 1118, and a −x displacedslit pattern 1116 with reference to undisplaced slit pattern locations1100, 1102, 1104, 1108, 1106, respectively. As is apparent, beammodulations associated with small pattern shifts in the x-direction canbe made negligible with sufficiently long, narrow slits. FIGS. 11B-11Cshow corresponding displacements of diagonally configured slit patterns.In the examples of FIGS. 11B-11C, elongated, parallelogram-shaped slitpatterns are oriented at +60 degrees and −60 degrees, with respect tothe scan direction but having the same x-direction height at the patternof FIG. 11A. Orientations of the slit patterns at angles other than 60degrees can also be used. Relative distortion contributions can beestimated based on a long dimension of the pattern projected parallel toa scan direction. Contributions associated with short slit dimensions(along the y-axis in this example) can be discounted as slits areassumed to be narrow. In other examples, slit patterns can be arrangedat other orientations. By comparing relative temporal offsets associatedwith differently oriented slits, a magnitude and a direction ofdistortion can be estimated. Since distortion is characterized by twoquantities, magnitude and direction, three measurements, eachmeasurement based on different slit orientations, are sufficient,although additional measurements with slits in other directions may beuseful as well. Table 1 lists distortion contributions associated withsimultaneous measurements of the temporal offsets in differentdistortion directions for the slit configurations of FIGS. 11A-11C inarbitrary units.

Relative distortion contributions associated with slit patterns arrangedin different orientations Distortion direction relative to stagedirection Slit 1 Slit 2 Slit 3 0 4 4 4 +45 deg 2.83 4.46 1.27 −45 deg2.83 1.27 4.46  90 deg 0 2.31 −2.31In the above table, it was assumed that slit patterns 1, 2, and 3 weredisplaced and that associated interrogation signals were captured atsubstantially the same time. An implicit assumption is that thedistortion is substantially uniform over the region of the reticleexposed by the array of slit images, so that each slit experiencessubstantially the same distortion. Alternatively, the slits can be atthe same x-location and experience a displacement in the y-direction.The group of slits can sample the same reticle region by capturingsignals at sequential times when each slit sees the same reticle region.If the stage moves additionally in the x-direction, the responses of theslit detectors to distortions of different magnitudes and directions canbe calibrated using precisely controlled stage motions in differentdirections.

Other illumination patterns can be used as well. For example, FIGS.11D-11I show an elongated diamond-shaped pattern 1130 arranged in twodifferent orientations with various reticle displacements such as shownin FIGS. 11A-11C. Such a pattern or other asymmetric pattern (orcombinations of such patterns) can be used in estimating distortiondirection and magnitude. Patterns that are asymmetric generally permitevaluation of displacements and distortion in a scan direction and adirection perpendicular to a scan direction.

Direct Measurement of Distortion

A speckle interferometry apparatus 1000 as illustrated in FIG. 10, maybe used to detect distortion of a reticle 1002 secured to a moveablestage 1004. The speckle interferometry apparatus 1000 includes a firstinterrogation beam source 1010 that is configured to direct a firstinterrogation beam 1011 along a first interrogation axis 1012 at anangle of incidence θ_(i) to a chrome reticle pattern 1008. Alternativelythe interrogation beams may be directed to the side of the reticleobverse to the chrome pattern, and the speckle pattern created fromsmall variations in the reticle substrate surface finish. A secondinterrogation beam source 1013 is configured to direct a secondinterrogation beam 1014 along a second interrogation axis 1015 at anangle of incidence θ_(i) to the chrome reticle pattern 1008 so as thatat least portions of the first and second interrogation beams overlap atthe reticle pattern 1008, and the planes of incidence of beams 1011 and1014 are the same. The two interrogation beams have a fixed coherentrelation to one another and are preferably derived from a single laser.A detector 1016 is situated along an axis 1017 and is configured toreceive portions of the first and second interrogation beams from thereticle 1002 and produce a detected signal corresponding to a combinedspeckle interference pattern associated with speckle produced by each ofthe interrogation beams. A lens 1024 is situated long the axis 1017 soas to image the reticle pattern 1008 on the detector 1016. One or moreother optical elements (e.g., lenses, mirrors, and the like) may beplaced in front of the detector 1016 to, for example, increase radiationcollection efficiency. Phase shift elements 1030, 1031 are situated onthe axes 1012, 1015 so as to modulate the phase of the beams 1011,respectively. In the following, two assumptions are made: the amount ofreticle distortion created is smaller than a typical speckle feature;and the amount of distortion does not vary over the field of view of thedetector 1016.

The intensity of the speckle pattern at the detector 1016 can be writtenasI=A+B cos(Ψ),wherein A, B are intensities associated with the portions of the firstand second interrogation beams directed to the detector 1016, and Ψ is aphase term that varies from point to point on the reticle 1002 butremains constant at a given time during the scan absent reticledeformation. If the reticle 1002 is displaced laterally by a distance Δywhich lies in the plane of incidence of light beams 1011 and 1014, andis preferably aligned with the reticle stage scan direction, there is anadditional phase term 2kΔy sin(θ_(i)) so that the intensity becomes:I=A+B cos(Ψ+2kΔy sin(θ_(i))),wherein k=2π/λ.

The phase terms and Ψ and 2kΔy sin(θ_(i)) can be measured using phaseshift interferometry. A plurality of calibration phase shifts φ_(i) areintroduced into one of the two incident beams (1011, 1014) by arespective one of the phase modulators 1030, 1031 and the associatedintensities I_(i) at the detector 1016 recorded. In the absence ofreticle distortion, Δy=0. The phase term Ψ is then found from theequation below:

${\Psi = {\tan^{- 1}\frac{\sum\limits_{i}{\alpha_{i}I_{i}}}{\sum\limits_{i}{\beta_{i}I_{i}}}}},$wherein the values α_(i) and β_(i) can be determined from phase shiftinterferometry tables. Generally α and β_(i) depend on a number of phaseshifts used and magnitudes of the phase shifts. A convenient and commonselection for phase shifts is four measurements (i=1, 2, 3, 4) withphase increments of π/2, with corresponding measured intensities I₁, I₂,I₃, I₄ which leads to the following relation:

$\Psi = {\tan^{- 1}{\frac{\left( {I_{4} - I_{2}} \right)}{\left( {I_{1} - I_{3}} \right)}.}}$With the phase term determined, the displacement dependent phase term2kΔy sin(θ_(i)) can be determined as a difference between phases withand without displacement. With this method, displacements caused bythermal distortion are substantially indistinguishable fromdisplacements associated with reticle stage motion so that interrogationbeam detection and signal recording should be carefully timed so thatreticle stage motion induced phase contributions do not appear asthermal distortions. The calibration phase shifts φ_(i) are normallyapplied rapidly, to reduce the effects of vibration and air temperaturefluctuations. Because of the scanning of the reticle stage, measurementswith for each of the calibration phase increments will be made atsomewhat different reticle locations. Even with phase shifting at ratesas high as 1 MHz, at each phase increment, reticle locations areseparated by about 1 μm at a reticle stage speed V=1 m/s. Thus, suchphase shift calibration measurements can include significant,undesirable Δy dependent phase contributions. Therefore, in someexamples, phase shift calibration measurements are made duringconsecutive scans (or other different scans such as alternating scans)at prescribed times at which the reticle stage 1004 and the reticle 1002are in substantially the same position. During these scans the phasestability of the interferometry apparatus 1000 must be maintained inorder to accurately acquire the calibration phase shifts φ_(i).

With this technique the component of distortion Δy parallel to the planeof incidence of the interrogation beams 1011 and 1014 is determined. Byutilizing a second interferometry apparatus with interrogation beamsoriented so that their planes of incidence are perpendicular to those ofthe first interferometry apparatus, a component of the distortion Δx canalso be measured. Thus by combing the results of the two apparatus, areticle distortion of arbitrary direction can be determined from its twocomponents Δx and Δy.

Additional Considerations

The methods and apparatus described herein permit assessment of reticledistortions based on the detection and processing of reticle modulatedportions of an interrogation flux. Such modulated fluxes can beassociated with electrical signals having significant reticledeformation information in signal changes in time periods as short as afew microseconds or less. Depending on a reticle scan speed, signalvariations in microsecond time frames may be associated with distortionmeasurements accurate to about 1 nm at sampling rates of about 1Gsample/s, with a statistical error in the range of about 1-5%. Howeverat a 1 Gsample/s sampling rate, a 1% statistical error in the detectedsignal means the signal count rate is (1/.01)^2*10⁹=10¹³ Hz. For 500 nmphotons (˜2.5 eV), the associated beam power is 2.5*1.6×10⁻¹⁹*10¹³=4 μWat the detector. Based on a detector collection efficiency of about0.1%, the power incident on the reticle during a measurement is 4 mW. Ifthe interrogation flux is turned off between measurements, the resultingheat load to the reticle is likely to be negligible. By comparison, theaverage power absorbed by the reticle during exposure (i.e., duringoperation of the lithography device) exceeds 4 W. However, the powerdensity from the interrogation beam can be much higher than that duringexposure, e.g., if the interrogation beam irradiates a slit pattern onthe reticle of area 10 μm×100 μm, the power density is 400 W/cm². Thisis much higher than during exposure, but it is still significantly belowthe level of intensity that can cause damage.

In the examples above, reticle stage and reticle motion is generallyassumed to be one-dimensional, so that when the stage is at a givenlocation, the reticle is at an associated location exclusive of anyinduced distortion. However, the reticle stage may not return thereticle to the same location from scan to scan and the reticle may havedifferent positions and orientations relative to one or moreinterrogation beam detectors, even if the reticle stage returns to thesame center-of-mass position. Reticle stage height can also becontrolled to vary so as to increase the effective depth of focus (DOF)at the wafer. Since stage DOF corrections are generally measured in mostlithographic processing, corresponding corrections can be applied tointerrogation beam detector signals. Changes to detector signals tend tobe associated with reticle stage z-axis motion, or rotations of thereticle stage in a plane defined by a vertical axis and a propagationdirection to an interrogation beam detector. Both of these motionsproduce vertical displacements of the interrogation beam portions at thedetector. In one embodiment, the detector is mounted on an actuator thatmoves vertically according to a signal from the stage controller tocorrect for or partially compensate such stage motions. Detectorresponse variations can also be calibrated using fixed patterns mountedon the reticle stage. Complications related to stage positioning can beavoided if distortion measurements are made during a dedicated scancarried out at a prescribed height and orientation. This can be done,for example, during wafer exchange.

Precision Systems

The methods and apparatus disclosed above can be used in conjunctionwith various precision systems such as various types of lithographysystems and other wafer processing systems and methods. Turning to FIG.13, certain features of an immersion lithography system (an exemplaryprecision system) are shown, namely, a light source 1340, anillumination-optical system 1342, a reticle stage 1344, aprojection-optical system 1346, and a wafer (substrate) stage 1348, allarranged along an optical axis A. The light source 1340 is configured toproduce a pulsed beam of illumination light, such as DUV light of 248 nmas produced by a KrF excimer laser, DUV light of 193 nm as produced byan ArF excimer laser, or DUV light of 157 nm as produced by an F₂excimer laser. The illumination-optical system 1342 includes an opticalintegrator and at least one lens that conditions and shapes theillumination beam for illumination of a specified region on a patternedreticle 1350 mounted to the reticle stage 1344. The pattern as definedon the reticle 1350 corresponds to the pattern to be transferredlithographically to a wafer 1352 that is held on the wafer stage 1348.Lithographic transfer in this system is by projection of an aerial imageof the pattern from the reticle 1350 to the wafer 1352 using theprojection-optical system 1346. The projection-optical system 1346typically comprises many individual optical elements (not detailed) thatproject the image at a specified demagnification ratio (e.g., 1/4 or1/5) on the wafer 1352. So as to be imprintable, the wafer surface iscoated with a layer of a suitable exposure-sensitive material termed a“resist.”

The reticle stage 1344 is configured to move the reticle 1350 in theX-direction, Y-direction, and rotationally about the Z-axis. To suchend, the reticle stage is equipped with one or more linear motors havingcooled coils as described herein. The two-dimensional position andorientation of the reticle 1350 on the reticle stage 1344 are detectedby a laser interferometer (not shown) in real time, and positioning ofthe reticle 1350 is effected by a main control unit on the basis of thedetection thus made.

The wafer 1352 is held by a wafer holder (“chuck,” not shown) on thewafer stage 1348. The wafer stage 1348 includes a mechanism (not shown)for controlling and adjusting, as required, the focusing position (alongthe Z-axis) and the tilting angle of the wafer 1352. The wafer stage1348 also includes electromagnetic actuators (e.g., linear motors or aplanar motor, or both) for moving the wafer in the X-Y planesubstantially parallel to the image-formation surface of theprojection-optical system 1346. These actuators desirably compriselinear motors, one more planar motors, or both.

The wafer stage 1348 also includes mechanisms for adjusting the tiltingangle of the wafer 1352 by an auto-focusing and auto-leveling method.Thus, the wafer stage serves to align the wafer surface with the imagesurface of the projection-optical system. The two-dimensional positionand orientation of the wafer are monitored in real time by another laserinterferometer (not shown). Control data based on the results of thismonitoring are transmitted from the main control unit to a drivecircuits for driving the wafer stage. During exposure, the light passingthrough the projection-optical system is made to move in a sequentialmanner from one location to another on the wafer, according to thepattern on the reticle in a step-and-repeat or step-and-scan manner.

The projection-optical system 1346 normally comprises many lens elementsthat work cooperatively to form the exposure image on the resist-coatedsurface of the wafer 1352. For convenience, the most distal opticalelement (i.e., closest to the wafer surface) is an objective lens 1353.Since the depicted system is an immersion lithography system, itincludes an immersion liquid 1354 situated between the objective lens1353 and the surface of the wafer 1352. As discussed above, theimmersion liquid 1354 is of a specified type. The immersion liquid ispresent at least while the pattern image of the reticle is being exposedonto the wafer.

The immersion liquid 1354 is provided from a liquid-supply unit 1356that may comprise a tank, a pump, and a temperature regulator (notindividually shown). The liquid 1354 is gently discharged by a nozzlemechanism 1355 into the gap between the objective lens 1353 and thewafer surface. A liquid-recovery system 1358 includes a recovery nozzle1357 that removes liquid from the gap as the supply 1356 provides freshliquid 1354. As a result, a substantially constant volume ofcontinuously replaced immersion liquid 1354 is provided between theobjective lens 1353 and the wafer surface. The temperature of the liquidis regulated to be approximately the same as the temperature inside thechamber in which the lithography system itself is disposed.

Also shown is a sensor window 1360 extending across a recess 1362,defined in the wafer stage 1348, in which a sensor 1364 is located.Thus, the window 1360 sequesters the sensor 1364 in the recess 1362.Movement of the wafer stage 1348 so as to place the window 1360 beneaththe objective lens 1353, with continuous replacement of the immersionfluid 1354, allows a beam passing through the projection-optical system1346 to transmit through the immersion fluid and the window 1360 to thesensor 1364.

An interrogation beam source 1380 is situated to direct an interrogationoptical beam 1381 to the reticle 1350, and a detection system 1382 isconfigured to detect a portion of the interrogation beam as modulated bythe reticle 1351. The detected beam can be used as described above toassess reticle distortion so that suitable system adjustments can bemade to correct, prevent, or at least partially compensate distortion.

Referring now to FIG. 14, an alternative embodiment of a precisionsystem that can include one or more electromagnetic actuators havingactively cooled coils as described herein is an EUVL system 1400, as arepresentative precision system incorporating an electromagneticactuator as described herein, is shown. The depicted system 1400comprises a vacuum chamber 1402 including vacuum pumps 1406 a, 1406 bthat are arranged to enable desired vacuum levels to be established andmaintained within respective chambers 1408 a, 1408 b of the vacuumchamber 1402. For example, the vacuum pump 1406 a maintains a vacuumlevel of approximately 50 mTorr in the upper chamber (reticle chamber)1408 a, and the vacuum pump 1406 b maintains a vacuum level of less thanapproximately 1 mTorr in the lower chamber (optical chamber) 1408 b. Thetwo chambers 1408 a, 1408 b are separated from each other by a barrierwall 1420. Various components of the EUVL system 1400 are not shown, forease of discussion, although it will be appreciated that the EUVL system1400 can include components such as a reaction frame, avibration-isolation mechanism, various actuators, and variouscontrollers.

An EUV reticle 1416 is held by a reticle chuck 1414 coupled to a reticlestage 1410. The reticle stage 1410 holds the reticle 1416 and allows thereticle to be moved laterally in a scanning manner, for example, duringuse of the reticle for making lithographic exposures. Between thereticle 1416 and the barrier wall 1420 is a blind apparatus. Anillumination source 1424 produces an EUV illumination beam 1426 thatenters the optical chamber 1408 b and reflects from one or more mirrors1428 and through an illumination-optical system 1422 to illuminate adesired location on the reticle 1416. As the illumination beam 1426reflects from the reticle 1416, the beam is “patterned” by the patternportion actually being illuminated on the reticle. The barrier wall 1420serves as a differential-pressure barrier and can serve as a reticleshield that protects the reticle 1416 from particulate contaminationduring use. The barrier wall 1420 defines an aperture 1434 through whichthe illumination beam 1426 may illuminate the desired region of thereticle 1416. The incident illumination beam 1426 on the reticle 1416becomes patterned by interaction with pattern-defining elements on thereticle, and the resulting patterned beam 1430 propagates generallydownward through a projection-optical system 1438 onto the surface of awafer 1432 held by a wafer chuck 1436 on a wafer stage 1440 thatperforms scanning motions of the wafer during exposure. Hence, images ofthe reticle pattern are projected onto the wafer 1432.

The wafer stage 1440 can include (not detailed) a positioning stage thatmay be driven by a planar motor or one or more linear motors, forexample, and a wafer table that is magnetically coupled to thepositioning stage using an EI-core actuator, for example. The waferchuck 1436 is coupled to the wafer table, and may be levitated relativeto the wafer table by one or more voice-coil motors, for example. If thepositioning stage is driven by a planar motor, the planar motortypically utilizes respective electromagnetic forces generated bymagnets and corresponding armature coils arranged in two dimensions. Thepositioning stage is configured to move in multiple degrees of freedomof motion, e.g., three to six degrees of freedom, to allow the wafer1432 to be positioned at a desired position and orientation relative tothe projection-optical system 1438 and the reticle 1416.

An EUVL system including the above-described EUV-source andillumination-optical system can be constructed by assembling variousassemblies and subsystems in a manner ensuring that prescribed standardsof mechanical accuracy, electrical accuracy, and optical accuracy aremet and maintained. To establish these standards before, during, andafter assembly, various subsystems (especially the illumination-opticalsystem 1422 and projection-optical system 1438) are assessed andadjusted as required to achieve the specified accuracy standards.Similar assessments and adjustments are performed as required of themechanical and electrical subsystems and assemblies. Assembly of thevarious subsystems and assemblies includes the creation of optical andmechanical interfaces, electrical interconnections, and plumbinginterconnections as required between assemblies and subsystems. Afterassembling the EUVL system, further assessments, calibrations, andadjustments are made as required to ensure attainment of specifiedsystem accuracy and precision of operation. To maintain certainstandards of cleanliness and avoidance of contamination, the EUVL system(as well as certain subsystems and assemblies of the system) areassembled in a clean room or the like in which particulatecontamination, temperature, and humidity are controlled.

As shown in FIG. 14, an interrogation beam source 1450 can be situatedso as to direct an interrogation optical beam 1451 to the reticle 1416.A detection system 1452 is situated to receive at least a portion of theinterrogation beam that is reflected, refracted, diffracted,phase-shifted or otherwise modulated by interaction with the reticle1416. Based on a detector signal response to this beam portion, reticledistortion can be assessed as described above in the detection system.

Semiconductor devices can be fabricated by processes includingmicrolithography steps performed using a microlithography system asdescribed above. Referring to FIG. 15, in step 1501 the function andperformance characteristics of the semiconductor device are designed. Instep 1502 a reticle (“mask”) defining the desired pattern is designedand fabricated according to the previous design step. Meanwhile, in step1503, a substrate (wafer) is fabricated and coated with a suitableresist. In step 1504 (“wafer processing”) the reticle pattern designedin step 1502 is exposed onto the surface of the substrate using themicrolithography system. In a step 1510, reticle distortion can beestimated during exposure as described above. In step 1505 thesemiconductor device is assembled (including “dicing” by whichindividual devices or “chips” are cut from the wafer, “bonding” by whichwires are bonded to particular locations on the chips, and “packaging”by which the devices are enclosed in appropriate packages for use). Instep 1506 the assembled devices are tested and inspected.

Representative details of a wafer-processing process including amicrolithography step are shown in FIG. 16. In step 1611 (“oxidation”)the wafer surface is oxidized. In step 1612 (“CVD”) an insulative layeris formed on the wafer surface by chemical-vapor deposition. In step1613 (electrode formation) electrodes are formed on the wafer surface byvapor deposition, for example. In step 1614 (“ion implantation”) ionsare implanted in the wafer surface. These steps 1611-1614 constituterepresentative “pre-processing” steps for wafers, and selections aremade at each step according to processing requirements.

At each stage of wafer processing, when the pre-processing steps havebeen completed, the following “post-processing” steps are implemented. Afirst post-process step is step 1615 (“photoresist formation”) in whicha suitable resist is applied to the surface of the wafer. Next, in step1616 (“exposure”), the microlithography system described above is usedfor lithographically transferring a pattern from the reticle to theresist layer on the wafer. Reticle distortion can be compensated duringpattern transfer. In step 1617 (“developing”) the exposed resist on thewafer is developed to form a usable mask pattern, corresponding to theresist pattern, in the resist on the wafer. In step 1618 (“etching”),regions not covered by developed resist (i.e., exposed materialsurfaces) are etched away to a controlled depth. In step 1619(“photoresist removal”), residual developed resist is removed(“stripped”) from the wafer.

Formation of multiple interconnected layers of circuit patterns on thewafer is achieved by repeating the pre-processing and post-processingsteps as required. Generally, a set of pre-processing andpost-processing steps are conducted to form each layer.

The disclosed methods and apparatus can be applied to the estimation anddetection of reticle distortions in lithographic systems such asdescribed above. An optical interrogation beam can be directed to areticle, and portions of such a beam that are reflected, refracted,diffracted, scattered or otherwise captured by a detector can be used toproduce electrical signals indicative of reticle deformation. Typicallyelectrical signals obtained from two or more scans are used, and theinterrogation beam can be patterned a magnitude and a direction or thedistortion can be estimated.

The above examples are provided in order to illustrate selectedembodiments, but the invention is not to be limited by features in anyparticular embodiment. I claim all that is encompassed by the appendedclaims.

The invention claimed is:
 1. A method for detecting reticle patterndistortion, the method comprising: (a) directing at least one opticalbeam to be incident on the reticle as the reticle undergoes a firstscanning motion; (b) detecting at least one segment of a secondary lightsignal associated with an interaction of the at least one optical beamwith the reticle during the first scanning motion; (c) directing atleast one optical beam to be incident on the reticle as the reticleundergoes a second scanning motion, the second scanning motion performedat a time different than a time associated with the first scanningmotion; (d) detecting at least one segment of a secondary light signalassociated with an interaction of the at least one optical beam and thereticle during the second scanning motion; and (e) based on the detectedsegments associated with the first scanning motion of the reticle andthe second scanning motion of the reticle, determining that the reticlepattern is distorted.
 2. The method of claim 1, wherein thedetermination is based on a comparison of at least one segment among thesegments detected during the first scanning motion of the reticle andthe second scanning motion of the reticle, with a reference segment. 3.The method of claim 2, further comprising directing an exposure beam tothe reticle during acquisition of the detected segment.
 4. The method ofclaim 2, wherein the at least one segment is detectedinterferometrically.
 5. The method of claim 4, further comprisingproviding a reference optical beam, wherein the at least one segment isdetected interferometrically based on interference with the referenceoptical beam.
 6. The method of claim 3, wherein the at least one segmentis detected based on at least one detected speckle pattern.
 7. Themethod of claim 3, further comprising estimating at least one of amagnitude and a phase of a distortion.
 8. The method of claim 7, whereinthe estimate is based on a temporal shift between the detected segmentand the reference segment.
 9. The method of claim 1, further comprising:repetitively detecting segments of secondary light signals associatedwith an interaction of the at least one optical beam and the reticleduring at least portions of a series of scan intervals, whereindetermining the reticle pattern is distorted based on comparisons of thedetected segments.
 10. The method of claim 9, further comprisingestimating at least one of distortion magnitude and direction based onthe comparison.
 11. The method of claim 9, wherein the comparisons arebased on correlation of detected segments.
 12. The method of claim 9,wherein the segments are detected based on optical interference orspeckle.
 13. An apparatus, comprising: an optical radiation sourceconfigured to produce a patterned interrogation beam and direct thepatterned beam to a reticle plane; a detection system situated toreceive at least a portion of the patterned interrogation beam from thereticle plane and produce a detected optical signal; and a processingsystem coupled to the detection system, the processing system providingan indication of pattern distortion at the reticle plane based on avariation in the detected optical signal as a function of time.
 14. Themethod of claim 13, wherein the patterned interrogation beam isasymmetric with respect to a reticle scan direction.
 15. The apparatusof claim 13, wherein the processing system is configured to estimate adistortion direction based on the asymmetry of the patternedinterrogation beam.
 16. The apparatus of claim 13, wherein the patternedinterrogation is elongated along a scan direction or in a directionorthogonal to the scan direction.
 17. The apparatus of claim 13, whereinthe patterned interrogation beam has a slit pattern that is elongatedparallel to or at an angle of 45 degrees with respect to the scandirection.
 18. The apparatus of claim 13, wherein the detector isconfigured to detect the portion of the patterned interrogation beam bydirect detection or phase-sensitive detection.
 19. The apparatus ofclaim 13, wherein the detector is configured to detect the portion ofthe patterned interrogation beam as a speckle pattern, and the processoris configured to estimate pattern distortion magnitude and directionbased on the speckle pattern.
 20. A lithographic system for transferringa pattern from a reticle to a sensitized substrate, comprising theapparatus of claim
 13. 21. A lithographic method, comprising: detectinga reticle pattern distortion according to the method of claim 1; andadjusting at least one lithographic system parameter based on thedetected reticle pattern distortion.
 22. A lithographic method,comprising: detecting a reticle pattern distortion according to themethod of claim 9; and adjusting at least one lithographic systemparameter based on the detected reticle pattern distortion.